This invention relates to a compensatory circuits for electrostatic printers.
Known in the art are electrostatic printers of the type having a recording station and a development station through which a recording medium is passed. An example of such a printer is disclosed in U.S. Pat. No. 3,729,123. The recording medium comprises a dielectric portion supported by a substantially conductive backing portion. The dielectric portion supports a latent electrostatic image formed at the recording station which is then developed by a toner medium at the development station.
The latent image is formed by a plurality of recording styli or stylus type electrodes physically positioned to electrically address the dielectric portion of the recording medium as the medium travels through the recording station. Diametrically opposite to the stylus electrodes in a series of backup electrodes. These oppositely opposed stylus and backup electrodes form a printing gap. An electrostatic charge is deposited on the dielectric portion of the moving recording medium when the potential difference between an addressed stylus electrodes and the oppositely opposed backup electrodes is raised to a threshold level, usually 500 volts or more in most types of such printers. The passage of the recording medium through the development station renders the deposited charge visible. A liquid medium having suspended toner particles is presented to the surface of the dielectric portion with the deposited electrostatic image and toner particles are attached to an adhere to the electrostatic charged portions of the recording medium.
As exemplified in U.S. Pat. No. 3,729,123, in printing (forming electrostatic latent images), the addressed stylus electrodes are connected to ground while oppositely opposed backup electrodes are raised to large writing potentials of 500 volts or more.
The raising of substantially instantaneous potentials from zero to 500 volts on these electrodes requires expensive circuit drivers for the backup electrodes.
In order to reduce the requirements of such circuits and also increase the available writing time, the circuit drivers for stylus and backup electrodes are maintained at a bias level above ground. Modulating from a nonprinting to printing mode is done from the bias level to either a higher voltage level or to ground. For example, the backup electrodes may be maintained at plus 200 voltages and modulated for writing to plus 600 volts. At the same time the addressed stylus electrodes may be modulated from plus 200 voltages to zero. Shorter time requirements are obtainable in modulating these comparatively lower potential levels and less powerful and sophisticated electronic drivers are needed for this task. All that is necessary is single voltage supply. The necessary voltage amplitude modulation can easily be derived from the same power supply. Furthermore, the number of components is minimized and voltage translation requirements are minimized. Direct interfacing to control logic circuitry can be easily accomplished.
Examples of types of other above ground maintenance for non-printing and printing mode functions are illustrated in the electrode drivers and circuits of U.S. Pat. Nos. 3,569,983 and 3,958,251.
Although these desirable advantages are obtained in employing such above ground printing techniques, a varying potential level always exists across the printing gap due to the presence of above ground potentials. Although these potentials may not be sufficient to deposit an electrostatic charge on the recording medium during the nonprinting mode, they are sufficient to present problems elsewhere in the printer system. This varying but established potential creates a current flow in the conductive backing portion of the recording medium. The current in turn provides potentials above ground at points in the development station where the recording medium contacts the back-support or rest positioned opposite to the toner dispenser. Even though the back-support may be connected to ground reference as shown in U.S. Pat. No. 3,729,123, potentials above ground exist in the conductive portion of the medium at the development station due to contact impedance established between the back-support and the record medium. These potentials create electric fields strong enough to cause toner particles to "plate out" of the toner carrier medium and deposit on the surface of the toner dispenser. Through the passage of time, toner particles build up to an undesirable degree on the dispenser surface interferring with proper toning of electrostatic image on the dielectric portion of the medium.
Another problem that occurs in these printing systems is the development of triboelectric charge on the recording medium. This triboelectric charge is caused by the frictional drag of the recording medium over the stylus electrode dielectric head support. This charge creates an undesirable developed background in nonimage areas of the developed image during passage through the development station.